1865
Stable Carbocations.
CXL.'
Naphthalenium Ions
George A. Olah,* Gheorghe D. Mateescu, and Y. K. Mo
Contribution from the Department of Chemistry, Case Western Reserve University, Cleveland, Ohio 44106. Received August 4, 1972 Abstract: Protonation of naphthalene, methyl-, halo-, hydroxy-, and methoxynaphthalenes in superacid (HF-
SbF5-SO2C1F or SbFS-FSOsH-SO:CIF) solutions leads to the formation of the stable naphthalenium ion and methyl-, halo-, hydroxy-, and methoxynaphthalenium ions, respectively. The direct observation of these arenium ions by means of nmr spectroscopy is reported. The pmr spectrum of naphthalenium ion is temperature dependent indicating the presence of rapidly equilibrating ions (1-protonated and 2-protonated naphthalenium ions) uia simple, 1,2-hydrogen shift. The activation energy, E,, of 14.5 i 0.9 kcal/mol and the preexponential factor, A , of 1016*0.*sec-1 were estimated for this process. Protonation of isomeric methyl-, fluoro-, hydroxy-, and methoxynaphthalenes in superacid gave only one kind of ion, while from chloro-, bromo-, and iodonaphthalenes, two isomeric ions were formed. These results are discussed in terms of halogen size and proton affinity, as well as in terms of a repulsive interaction of the p (halogen) and T (ring) electrons. Substantial peri effect was observed in 1-halo-, -hydroxy-, and methoxynaphthalenium ions. The carbon-13 Fourier-transform nmr spectra of 1,5dimethylnaphthalene and its corresponding ion are also reported.
A
s expected, in the series of homologous arenes, benzene (with its ideal "aromatic sextet") is the most difficult to protonate. The "frozen" benzenium ion 1 can be observed only in the strongest superacid media2 and only at temperatures below - 130". Anthracene derivatives (for example, 9, lo-dimethylanthracene) easily give anthracenium ions 2 even in CF3COOH-Hz0-BF3 solutions. There is no report in the literature on the direct observation of protonated naphthalene 3a. It was therefore of interest to study the
R 1
3a
2
naphthalenium ion in order to complete the series of simple parent arenium ions.3 The study of protonation of substituted naphthalenes is also of interest. It adds further insight into the chemistry of substituted arenium ions when compared with the previously described monoalkyl-, halo-, hydroxy-, and alkoxybenzenium ions. 3 , 4 Results and Discussion Naphthalene and methyl-, halo-, hydroxy-, and methoxynaphthalenes were protonated in a superacid HF-SbF, (1 : 1 molar ratio)-SOKlF solution. The pmr parameters of the protonated species are summarized in Table I. Naphthalenium Ion. The preparation of the stable naphthalenium ion 3a was achieved when naphthalene 3 was treated with fluoroantimonic acid (HF-SbF5 1 : 1 molar ratio) dissolved in SOzCIF (1 : 1 v/v) at low temperature. The naphthalenium ion 3a is almost as diffi-
cult to prepare as the benzenium ion. A saturated solution of naphthalene in S02C1F was carefully added in small portions to the vigorously stirred HF-SbF5SOzCIF or FS03H-SbF5-S02C1F solution at - 78 ". Development of a dark red color was immediately observed.6 Alternatively, the superacid solution was carefully added directly to naphthalene in an nmr tube which was cooled at -78". The mixture was then stirred at this temperature until a clear, deep red solution was formed. The 100-MHz pmr spectrum of the clear solution (Figure 1) is consistent with structure 3a of protonated naphthalene. One can easily recognize the methylene peak at 6 5.27.6 The ortho ( 2 ) and para (4) positions with respect to the protonation site are, as expected, the most deshielded ones. Both signals appear as doublets at 6 9.51 (J2,3 = 7.8 Hz) and 9.81 (J3,4 = 7.2 Hz), respectively, with further unresolved splittings. This type of proton-proton coupling has also been observed in other arenium i o m 3 r 4 All these data are consistent with the results of a nmr study of monoalkylbenzenium ions. 3 , 4 a The meta-proton (3) signal overlaps with those of the B-ring protons (5-8). The pmr spectrum of the naphthalenium ion 3a is temperature dependent (Figure 2). The methylene and the ortho-proton signals broaden with increasing temperatures. The ortho-proton absorption broadens more rapidly than the methylene proton absorption. N o change of para, meta, and B-ring proton absorptions is noticeable in the range - 83 to - 10". The temperature-dependent pmr spectra clearly indicate the presence 4 via simple 1,2of the rapidly equilibrating ions 3a hydrogen shift. This pattern has also been observed in other arenium ions3a4 and is in agreement with the chemistry of naphthalene. The 1,2-hydrogen shift is assumed to proceed through a three-center bond benzonium ion transition state 4b.7
(1) Part CXXXIX: G. A. Olah, H. Lin, and Y . K. Mo, J . Amer. Chem. SOC.,94, 3667 (1972). (2) G. A. Olah, R. H. Schlosberg, D. P. Kelly, and G. D. Mateescu, ibid., 92, 2546 (1970).
(3) A detailed discussion on arenium ions is presented by D. M. Brouwer, E. L. Mackor, and C. MacLean in "Carbonium Ions," Vol. 2, G. A. Olah and P. v. R. Schleyer, Ed., Wiley, New York, N. Y., 1970, p 837. (4) (a) G. A. Olah, R. H. Schlosberg, R. D. Porter, Y . I 1,3 (loa, 4 6 = 1.55) > 2.6 (sa, A6 = 1.95) > 1,4 (12, A6 = 1.98) > 2,3 (9a,A6 = 2.14). Halo-, Hydroxy-, and Methoxynaphthalenium Ions. 1-Substituted naphthalenes 13-X were smoothly protonated in either SbF5-HF-SO2C1F (1 : 1 molar ratio) or SbF5-FS03H-S02C1F (1 : 1 molar ratio) solution to give the stable naphthalenium ions 14-X and 15-X.
SbFb-HFSO-CIF, -78" or SbF,-FSO H-S0,CIF'
13-X
"
14-F,X = F (100%) 14-C1,x = C1(9%) 14-Br,X = Br (60%) 14-I,X = I(O%) 14-OH,X =OH(100%) 14-OCH3,X =OCHq (100%)
15-F,X = F (0%) 15421, X = C1(1%) 15-Br,X = Br (40%) 15-1,X = I(lOO%) 15-OH,X =OH (0%) IB-OCH,,X =OCHj(O%)
The nmr ('H and 19F) spectra of representative ions 14-F, 14-C1, and 14-OH are shown in Figures Sa-c, respectively. The methylene protons of ion 14-F show a slightly broadened doublet at 6 5.50 with 5JHF = 7 Hz. This kind of long-range coupling has also been observed in other fluoroarenium i0ns.l' The ortho proton displays a multiplet at 6 9.9 since it couples to the meta proton, CH2, and also the fluorine atom. The chemical shift (6 9.9) is similar to the ortho protons' in the fluorobenzenium ion." The Cs proton of the B ring is deshielded by a (fluorine) peri effect12and shows a doublet at 6 9.2 (JHF= 8 Hz). The I9F nmr spectrum of ion 14-F shows a multiplet at 24.20 (100.3 ppm lower field than the precursor). The fluorine chemical shift, however, is slightly smaller than that of fluorobenzenium ion (4 11.2) indicating partial charge delocalization into the B ring. The pmr spectrum of 1-chloronaphthalene in SbF5FS03H-SOsC1F solution at -60" shows that ion 14-Cl is formed (Figure 8b). The slightly broadened singlet at 6 5.83 is assigned to the methylene protons. The deshielded ortho proton displays a slightly broadened = 8 Hz). Similarly, the Cs proton doublet at 6 9.6 (JHH of the B ring is deshielded by a peri effect and shows a ~ 8 Hz). The presence of a doublet at 6 9.3 ( J H = shoulder on the methylene signal and that of a small doublet a t 9.7 (see arrow in Figure 8b) are tentatively interpreted as due to the formation of ion 15-C1(10 %), since the remainder of the absorption lines of ion 15-C1 are obscured by those of ion 14-C1. In the case of 1-bromonaphthalene, two isomeric ions 14-Br and 15-Br, in a ratio of 3 : 2, were formed. There are two methylene resonances at 6 5.0 and 5.4,respectively, in the pmr spectrum of 1-bromonaphthalene in SbF5-HF-S02C1F solution at -60". Due to the viscosity of the solution, only broad spectral lines are observed for the remainder of the protons. On the other hand, the pmr spectrum of 1-iodonaphthalene in SbF5HF-S02C1F solution shows a broadened singlet absorption at 6 5.4 indicating that only one iodonaphthalenium ion is formed. Due to the high viscosity of the system at -90°, only a poorly resolved spectrum
+
(11) G. A. Olah and T. E. Kiovsky, J . Amer. Chem. SOC.,89, 5692 (1967). (12) (a) R. H. Martin, N. Defay, and F. Greets-Evrard, Tetrahedron, 20, 1505 (1964); (b) G. 0. Dudek, Spectrochim. Acta, 19, 691 (1963).
Olah, Mateescu, Mo / Naphthalenium Ions
1872
3,5
H3d
a
b
C
Figure 9. (a) Proton and fluorine nmr spectra of protonated 2-fluoronaphthalene; (b) pmr spectrum of protonated 2-chloronaphthalene; and (c) pmr spectrum of 2-metpoxynaphthalene.
was obtained. At higher temperatures the methylene protons at 6 5.4 gradually disappeared (probably due t o deprotonation). The remainder of the spectrum shows a broadened absorption at 6 8.6. It is difficult to determine which isomeric ion (14-1 or 15-1) is formed. However, the absence of a doublet at 6 9.2 can rule out the para protonation (or ion 14-1 formation), because the Cs proton of ion 14-1 (if formed) should be deshielded substantially by the peri effect. Such an effect was found in other halonaphthalenium ions (14-X, X = F, C1, and Br). Electrophilic aromatic substitution of iodobenzene shows the preferential formation of the ortho isomer.l3 Since iodine is comparatively large, electrophiles can, in all probability, attack the iodine lone pair in the first step and then migrate to the nearest carbon. Indeed, stable alkylarylhalonium ions, in particular, iodonium ions, have been prepared and directly observed by means of nmr spectroscopy.14 Our present results seem to indicate the involvement of initial iodine protonation (kinetically controlled process) with subsequent C protonation. It should be noted that the ortho protonation or ion 15-X formation is enhanced with the increase of the size of the halogen. Furthermore, the isomeric ion distribution (14-X and 15-X) is temperature independent in the range -80 to -20' (thermodynamically controlled process). A repulsive interaction of the p (halogen) and i~ (ring) electrons (which would enhance the electron density at the para position in the order F > C1 > Br > 1 ) 1 5 may also explain this isomer distribution. Since hydroxy and methoxy groups are highly activating the aromatic rings, protonation of 1-hydroxy- and 1methoxynaphthalenes can be achieved even in weaker acid such as FSO2H-SO2C1F at -40 and -35", respectively. The pmr spectrum of ion 14-OH (Figure 8c) shows a slightly broadened singlet absorption at 6 4.7 for the methylene protons. The meta proton reveals a (13) G. A. Olah, S. J. Kuhn, and S. H. Flood, J . Atner. Chen7. SOC., 83, 4581 (1961). (14) G. A. Olah and E. Melby, ibid., 94, 6220 (1972). (1 5) L. M. Stock, "Aromatic Substitution Reaction," Prentice-Hall, Englewood Cliffs, N. J., 1968, p 56.
well-resolved doublet at 6 7.70 (Jo,m = 9 Hz). The ortho proton and the CS proton show complicated resonance lines centered at 6 8.8. Careful analysis shows that this region actually includes a doublet (ortho proton) and a doublet of triplets (C, proton). The substantial deshielding of the C8 proton indicates that the carbon-oxygen bond (C-OH) has a double bond (C=OH) character and thus a strong peri effect is observed. The pmr spectrum of ion 14-OCH3is similar to that of ion 14-OH except that an additional peak for OCHais observed at 6 5.04. Protonation of 2-substituted naphthalenes 16-X was also studied in various superacid media. The site of protonation is found to be at the carbons 1 and 3 to give ions 17-X and 18-X, respectively. The ratio of 17-Xj18-X depends on the nature of the substituent. SbF ,-HF (1:lmolar ratio) -SO,CIF
X
+
16-X
H
17-F,X-F (100%) 17-C1,X = C1(95%) 17-Br, X Br (90%) 17-1,X I(60%) 17.0H, X = OH(100%) 17-OCH3,X sOCH, (100%)
--
1&F,X = F (0%) 1W1,X C1(5%) l&Br, X = Br (10%) 18-1,X = I (40%) 18-OH,X =OH(O"/u) l&OCHB,X=OCH, (0%)
The nmr spectra of ions 17-F, 17-C1, and 17-OCH3 are shown in Figures 9a-c, respectively. The nmr parameters of ions 17-X and 18-X are summarized in Table 1. Protonation of 2-fluoronaphthalene in superacid HF-SbF5 (1 : 1 molar ratio)-SOzClF solution gave ion 17-F rather than 18-F. Evidence comes from the meta proton of ion 17-F showing the most shielded vinylproton triplet absorption at 6 8.03 (JHH= JHF= 8 Hz). If ion 18-F would have been formed, the meta proton
Journal of the American Chemical Society 1 95:6 1 March 21, 1973
1873
should have been a doublet. The coincidence of JHH and JHFin ion 17-F is not surprising since similar observations have been made in other fluorobenzenium ions.11 The para proton of ion 17-F shows multiplet resonance lines overlapping with that of the hydronium ion (H30+). The methylene protons are coupled to the fluorine and show a doublet at 6 5.4 (JHF= 8 Hz). The 19F nmr spectrum of ion 17-F shows a multiplet at 8.44 (deshielded by 98.9 ppm in comparison with the precursor, 16-F). The pmr spectra of 16-C1 and 16-Br when protonated in HF-SbF (1 : 1 molar ratio)-SOzCIF show the formation of ions 17-C1(95 %), 18-C1(5 %) and 17-Br (90 %), 18-Br (lo%), respectively. The para protons of ions = 8 Hz) 17-C1 and 17-Br show a doublet at 6 9.91 (JHH = 8 Hz), respectively. The methylene and 6 9.74 (JHH protons of both ions 17-C1 and 17-Br display a slightly broadened singlet at 6 5.5. In addition, there is a shoulder at 6 5.4 (e.g., Figure 9b, indicated by arrow) in the methylene proton region indicating the formation of the isomeric ions 18-C1 and 18-Br. Additional evidence for the formation of ions 18-C1 and 18-Br comes from the presence of a small broadened singlet at 6 9.5 (5%') and 10.0 (lox),respectively. These singlets are due to the ortho proton of ions 18-X (X = C1 and Br, respectively). Protonation of 2-iodonaphthalene in superacid SbFj-HF (1 :1 molar ratio)-SOzCIF gave two isomeric ions, 17-1 (60%) and 18-1 (40%). The pmr spectrum of the solution shows in the methylene proton region a broadened singlet at 6 5.2 and in the vinyl proton region a multiplet centered at 6 8.5, a doublet at 6 9.4 (JHH = 8 Hz), and a triplet at 6 9.2 (JHH = 2 Hz). The doublet at 6 9.4 clearly indicates that it is the para proton of ion 17-1 when compared with the spectra of ions 17-C1 and 17-Br. The triplet at 6 9.2 with a small coupling (0.5 Hz) is then assigned to the ortho proton of ion 18-1. The ratio of 17-1/18-I was determined by pmr integration and was found to be 312. Again no temperaturedependent pmr spectra were observed for ions 17-1 and 18-1 in superacid SbF,-HF (1 : 1 molar ratio)-SOzCIF solution in the range from - 80 to - 20". These results show that protonation of 2-halonaphthalenes in superacids is similar to that of i-naphthalenes. Initial protonation on halogen is favored in the order I > Br > Cl > F and subsequent proton transfer takes place from the n donor to the r donor.I6 Protonation and alkylation at the fluorine lone-pair level have not yet been observed in s ~ l u t i o n 'although ~ they have been demonstrated in the gas-phase (ion-molecular reactions). These data are also in agreement with our recent study of alkylation of bromo- and iodobenzenes lea ding to the formation of alkylarylhalonium ions.I4 Chloro- and fluorobenzenes do not undergo such reactions. Protonation of 2-hydroxy- and 2-methoxynaphthalenes in superacid gave exclusively ions 17-OH and 17-OCH3, respectively. The structure determination of ions 17-OH and 17-OCH3was based on their pmr spec= 9 Hz) indicates protra. A doublet at 6 9.51 (JHH
CH:
+
(16) Similar trends have been observed in the protonation of chloro and bromo substituted benzenes (see ref 3, sections 1I.C and V1.E). (17) G. A. Olah and J. R. DeMember, J . Amer. Chem. Soc., 91,2113 ( 1969).
(18) D . Holtz, J. C. Beauchamp, and S. D. Woodgate, ibid., 92,7478 ( 1970).
I
0
l
l
1
I
I
I
I
I
I
SO
I
I
I
I
I
I
IS0
100
I
I
PP.
from C f l
L
b Figure 10. Carbon-13 Fourier-transform nmr spectra of 1,5-dimethylnaphthalene 7 (a) and its protonated species (b).
tonation at carbon 3 with formation of ion 17-OH. Ion 18-OH was not observed. The pmr spectrum of protonated 2-methoxynaphthalene (17-OCH3) is temperature dependent (Figure 9c). The most deshielded one-proton triplet at 6 9.40 is due t o the overlapping of two doublets of the Ha proton in isomeric ions 17-OCHz-a and 17-OCH-b. At Hi,
I
CHJ
17-OCHJ-b
17-OCH3-a
higher temperature (ca. - 30") interconversion of 17-OCH3-a and 17-OCH-b is rapid and the triplet becomes broadened. Such behavior has been observed in the case of C-protonated anisole and its derivatives. 3,4b The meta proton of both ions 17-OCH3-a(b) shows a doublet at S 7.63 (JHH = 9 Hz) and is temperature independent. The shift is stronger for the Ha proton than for the meta proton, indicating strong ortho-para interaction. Carbon-13 Magnetic Resonance Studies. Since carbon-I3 magnetic resonance (cmr) reflects the electronic structure and charge distribution better than proton magnetic resonance, a 13C study of a selected naphthalenium ion 7a and its precursor 1,5-dimethylnaphthalene (7) has been carried out. As in the proton spectra, a dramatic change occurs in the carbon spectra upon protonation of the hydrocarbon. The differOlah, Mateescu, Mo / Naphthalenium Ions
1874
ences are even more remarkable because the relative spread of the chemical shifts in the case of I3Cis larger than for 'H. The electron-deficient carbon 1 in ion 7a exhibits a chemical shift (6 -7.4 ppm from I C s 2 ) which is deshielded by 64.5 ppm from its position in the neutral hydrocarbon (see Figure 10). A tentative assignment of the carbon absorptions is shown below and is compared with the cmr chemical shifts found for toluenium ion 19, revealing that the unprotonated ring preserves much of its aromatic character.4" @,9
+6*,o@+46,6
00
+m.1
+606 +53.i*"',.4.
+70.9
7
'
+;
114.9
,
t40.6 '\+1508
+145.1
19
7a
The application of the Fourier-transform natural abundance carbon nmr spectroscopy to the investigation of carbocation intermediates thus proves very useful and promises to open up a new dimension in our ability to elucidate in detail structures of even quite complicated and large ions. Experimental Section Materials. All the organic compounds used in this study were commercially available in high purity. Antimony pentafluoride
(Allied Chemical) was triply distilled; fluorosulfuric acid was doubly distilled. Preparation of Ions. The solution of naphthalenium ion was prepared as reported previously in this series for protonations in superacid media and studied directly by nmr spectrometry. The quenching of solutions of naphthalenium ions and the analysis of recovered hydrocarbons (by glc and pmr) were also carried out as described. 4% Nuclear Magnetic Resonance Measurement. Spectra were obtained on Varian Associates Model A56/60A, HA100, and HR300 spectrometers. Fourier-transform spectra were recorded either with a Bruker HX-90 nmr spectrometer (22.63 MHz) or with a Varian HA-100 FT nmr spectrometer (25.16 MHz). With the Bruker instrument lgF was used for locking the system (either C12C===CF2for low-temperature or C6F6 for room-temperature measurements). With the Varian instrument, a 13C lock (13Cenriched CHJ) was used for both low- and high-temperature measurements. In both cases a broad band (2 KHz) noise generator was used for proton noise decoupling.
Acknowledgment. Support of our work by the National Science Foundation and the Petroleum Research Fund, administered by the American Chemical Society, is gratefully acknowledged. We express our thanks to Dr. Toni Keller and Mr. Werner Schittenhelm (Bruker Scientific, Inc.) for the opportunity to record some of the early carbon-13 spectra with their instrument. The assistance of Dr. LeRoy F. Johnson and Mr. Lewis Cary (Varian Associates, Palo Alto, California) in recording the 300-MHz pmr spectra is acknowledged.
Radical Anions of 1,8-Bis-Unsaturated Naphthalene Derivatives Stephen F. Nelsen* and John P. Gillespie
Contribution f r o m the Department of Chemistry, University of Wisconsin, Madison, Wisconsin 53706. Received July 13, 1972 Abstract: The synthesis of 7,8-dimethylenenaphtho[lf,8']bicyclo[3.2.0]hept-2-ene and its conversion into 2,3dimethyl- and 2,3-cyclobutapleiadiene are described. The epoxide derived from naphtho[l',8']bicyclo[3.2.0]-
hepta-2,5-diene and the related hydrocarbon give the peri-bridged 1,8-divinylnaphthalenesresulting from cyclobutane cleavage upon pyrolysis. The esr spectra of the radical anions from six pleidaiene derivatives, 1-oxo-4naphtho[l',8'lcycloocta-2,4,7-triene, and 1,8-divinylnaphthaleneare reported and discussed. e report here the synthesis of some 1,8-bis-unsaturated derivatives of naphthalene, some limitations on the use of thermal cleavages of acenaphthene derivatives to prepare such compounds, and the esr spectra of several such radical anions. Experimental comparisons of the spin distributions of pleiadiene ( l ) , the related enol ether 2, and 1,8-divinylnaphthalene
W
a& 1
2
radical anions were of particular interest, to see if simple MO calculations would be successful for predictive purposes and, if not, to see what spin distributions actually result.
Journal o j t h e American Chemical Society
95:6
Synthesis. Several derivatives of pleiadiene (1) were prepared using the method of Meinwald and coworkers, who employed the heavy-atom solvent enhanced photocycloaddition of maleic anhydride to acenaphthylene to give 3a after work-up, lead tetraacetate decarboxylation to 4a, and pyrolytic cyclobutene ring opening to give l a . Starting the sequence with 3,6-di-tert-butylacenaphthenegave IC, and preparation of 3b by base-catalyzed exchange (KOD-DzO, 150") on 3a gave l b after completion of the sequence. We were unable to repeat the reported' 28% yields in the bis-decarboxylations and observed serious fallingoff of the yield in runs at a larger scale than 4 g. Attempts to improve conditions showed that saturation of the reaction mixtures with oxygen2 shortened the (1) J. Meinwald, G. E. Samuelson, and M. Ikeda, J . Amer. Chem. SOC.,92, 7604 (1970).
/ March 21, 1973
(2) C . M. Cimarsturi and J. Wolinsky, ibid., 90, 113 (1968).
